Distribution Point Units (DPU) serving subscribers with new copper access technologies for broadband communication services (e.g., G.fast) are typically deployed at a maximal distance of a few hundreds meters from the subscriber premises (e.g., 200 m for G.fast) in order to achieve the expected high data rates (e.g., 1 Gbps as aggregated data rates for G.fast). This means that the DPU will be located closer to the subscriber premises compared to current deployments. A local power supply to power the DPU is not always available at the DPU location. Therefore operators are requesting methods to feed the DPU via Reverse Power Feeding (RPF).
A Power Sourcing Equipment (PSE) installed at the subscriber premises provides a DC current to power the DPU via the same copper pair as used for broadband communication. The PSE is powered by the AC mains of the household. A Power Supply Unit (PSU) inside the DPU converts the DC voltage supplied by the PSE, typically 57V minus the DC voltage drop caused by the DC resistance along the copper pair, to a number of lower DC voltages used inside the DPU (e.g., 12V, 5V, 3V3, etc).
It is expected that operators will gradually upgrade their subscribers from a legacy communication service to the new broadband communication service provided by the DPU. These legacy communication services typically refer to the various Digital Subscriber Line flavors (xDSL), with or without the presence of Plain Old Telephone System (POTS). The legacy communication services are typically deployed from a central Office (CO) cabinet, so are for instance the ADSL or ADSL2+ communication services, from a Remote Unit (RU), so are for instance the VDSL2 communication services, or from a Local Exchange (LEX), so are for instance the POTS communication services.
At a given time, a particular subscriber is upgraded from the legacy communication service deployed from the legacy network equipment over the (long) legacy copper pair to the new broadband communication service deployed from the DPU over the (short) terminal segment of the copper pair.
To this end, a bypass switch is provisioned within the DPU. The bypass switch typically consists of a persistent relay with two switching states, namely:                the bypass state wherein the end user is transparently connected through the DPU to the legacy network equipment via the (long) legacy copper pair for support of the legacy communication service; or        the termination state wherein the (short) terminal segment of the copper pair (i.e., the segment spanning from the DPU down to the subscriber premises) is connected to a transceiver of the DPU for operation of the new broadband communication service, while the network segment of the copper pair (i.e., the segment spanning from the DPU up to the legacy network equipment) is disconnected and open-circuited at the DPU side.        
At the given time, the bypass relay shall thus be switched from the bypass state to the termination state for that particular subscriber. Preferably, this is accomplished without any human interaction (zero-touch installation).
Yet, there are a number of technical hurdles to overcome during the service upgrade.
Initially, when the very first user switches from a legacy communication service to the new broadband communication service, the DPU is unpowered since that user is also the very first user to feed the DPU. Hence, there is no power available at the DPU to switch the bypass relay either.
Furthermore, when a POTS service is present on the copper pair, no DC feed can be inserted on the copper pair by the PSE as this would conflict with the DC power source from the LEX (typically 48V, 53V or 60V). Although the DPU can directly get power from the latter, the DPU can only drain a little current during the ONHOOK state (typically from a few hundreds A up to a few mA) so as not to trigger an ONHOOK→OFFHOOK state transition and the insertion of the dial tone at the LEX, yielding a too long time to accumulate sufficient energy to switch the bypass relay and power its control logic.
If there is no POTS service delivered to the user, there is no DC voltage present on the copper pair either, and the PSE can start delivering DC power at once for the DPU to switch the bypass relay. However, some basic initialization protocol is still needed for the DPU to distinguish between DC power from the LEX with very little power available and DC power from the PSE with plenty of power available.